The pattern of switching of &lt;I&gt;S. pombe&lt;/i&gt; cells is nonrandom when assayed by single cell pedigrees. After two consecutive asymmetric cell divisions, one in four "granddaughter" cells undergoes a mating-type switch. Previously, we showed that this pattern is due to &lt;I&gt;mat1&lt;/i&gt; imprinting that marks only one sister chromatid in a strand-specific manner, and that is related to site-specific, double-stranded DNA break at &lt;I&gt;mat1&lt;/i&gt;. We now show that the imprint is a strand-specific, alkali-labile DNA modification at &lt;I&gt;mat1&lt;/i&gt;.&lt;BR&gt;&lt;BR&gt;The DNA break is an artifact, created from the imprint during DNA purification. We also proposed and tested the model that &lt;I&gt;mat1&lt;/i&gt;is preferentially replicated by a centromere-distal origin(s), so that the strand-specific imprint occurs only during lagging-strand synthesis. Altering the origin of replication, by inverting &lt;I&gt;mat1&lt;/i&gt;or introducing an origin of replication, affects the imprinting and switching efficiencies in predicted ways. Two-dimensional gel analysis confirmed that &lt;I&gt;mat1&lt;/i&gt;is preferentially replicated by a centromere-distal origin(s). Thus, the DNA replication machinery may confer different developmental potential to sister cells.&lt;BR&gt;&lt;BR&gt; Our recent work has discovered biochemical functions of &lt;I&gt;swi1&lt;/i&gt;and &lt;I&gt;swi3&lt;/i&gt;genes. We found that swi1p and swi3p perform imprinting by pausing and termination of DNA replication at &lt;I&gt;mat1&lt;/i&gt;. Our work shows (1) that the factors swi1p and swi3p act by pausing the replication fork at the imprinting site;and (2) that swi1p and swi3p are involved in termination at the &lt;I&gt;mat1&lt;/i&gt;-proximal polar-terminator of replication (&lt;I&gt;RTS1&lt;/i&gt;). A genetic screen to identify termination factors identified an allele that separated pausing/imprinting and termination of functions of swi1p. The results suggest that swi1p and swi3p promote imprinting in novel ways both by pausing replication at &lt;I&gt;mat1&lt;/i&gt;and by terminating replication at &lt;I&gt;RTS1&lt;/i&gt;. We also showed that Swi1 and Swi3 proteins form a complex in vivo and both bind to the RTS1 and the mat1 replication pause sites on the chromosome. Future studies are designed to define the mechanism of imprinting. We have defined a large number of mat1 mutations that affect imprinting. Their molecular analysis should help us define the mechanism of imprinting.